import/elpa: Add proper file-name to origin.

This avoids the lint "the source file name should contain the package
name" with generated packages.

Change-Id: I61c81203d9f3cdd035e2b64a4c7a8d86d1f3dd50
Signed-off-by: Zheng Junjie <zhengjunjie@iscas.ac.cn>

guix/import/elpa.scm

@@ -250,6 +250,7 @@ RECIPE."
        (uri (git-reference
              (url ,url)
              (commit ,commit)))
+       (file-name (git-file-name name version))
        (sha256
         (base32
          ,(bytevector->nix-base32-string